1. Field of the Invention
The present invention generally relates to magnetic disc drives, and more particularly to magnetic-field sensor devices comprising magnetic nanoparticles.
2. Description of the Related Art
Read-heads for magnetic disc drives currently sense magnetic bits by using an effect known as giant magnetoresistance (GMR). In essence, this type of sensor works because its resistance changes depending on the presence or absence of a magnetic field. The spatial resolution of such a device is limited by the dimensions of the active element in the sensor, which is currently defined lithographically. The GMR effect is described below.
One type of system which exhibits GMR is a magnetic tunnel junction. A magnetic tunnel junction is typically composed of two magnetic thin-film electrodes separated by a thin insulating layer. This insulating layer is sufficiently thin (˜5 nm or less) to allow electrons from one of the electrodes to move to the other electrode via quantum mechanical tunneling. Important to the operation of the junction is that the tunneling process preserve the spin of tunneling electrons.
For the case of two magnetic electrodes composed of identical materials, the tunneling conductance (G) for electrons to move through the junction is proportional to:G˜1+P2 cos θwhere P is the conduction electron polarization in the magnet (which is a measure of how magnetic the material is), and θ is the relative angle between electrode magnetic moments. The resistance of the junction is the inverse of its conductance.
If the magnetic moment of one electrode is flipped relative to the other electrode, then the tunneling rate through the junction changes. This resistance change, caused by a change in a magnetic moment orientation, can be used to sense magnetic fields in the manner shown in FIGS. 1(a) and 1(b). The sensor 1 is positioned in close proximity to a magnetic field source 2. The sensor is made of a short length of magnetic material 3 sandwiched between two magnetic electrodes 4 and 5. The magnetic material used to sense the magnetic field 2 is electrically isolated from the two electrodes 4 and 5 by insulating tunnel barriers 6 and 7. Additionally, the material 3 is chosen so that its magnetic moment changes orientation at fields below that of the two electrodes 4 and 5.
In operation, an electric current 8 is made to flow through the device. Initially, the magnetic moments of the sensor material 3 and electrodes 4 and 5 are parallel. When the sensor comes into close proximity with a magnetic field of sufficient strength, the moment of material 3 is flipped relative to the two electrodes 4 and 5 leading to a measurable change in the current through the device 8. The spatial resolution of such a sensor is directly related to the size of the sensor element 3. However, there is a need to improve the spatial resolution of such a magnetic field sensor, particularly in the area of read-heads for magnetic disc drives. Continually increasing storage densities demand the improved resolution.